الفهرس 
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Abstract
First-trimester risk assessment for fetal aneuploidy using nuchal translucency (NT) measurement is rapidly gaining popularity in the World. First introduced in England in the early 1990s, NT was established as a highly sensitive screening tool for Down syndrome with the publication of a large, multicenter trial in 1998. (Snijders et al.,
1998 )
In combination with maternal serum markers in the first trimester, the
screening performance is exceptionally good, with detection rates of more
than 80% with an approximately 5% rate of abnormal screening results
(screen positive rate of approximately 5% .) Recently, the method has
been validated for screening for Down syndrome and other aneuploidies
in multicenter trials in the United States and elsewhere. ( Malone et al
.,2011).
Patients are attracted to first trimester screening both for the
reassurance of highly reliable low-risk results as well as the privacy
afforded by early prenatal diagnosis. First-trimester screening offers a
significant improvement in the detection of aneuploidy, and with the
expanding role of first-trimester assessment for other complications of
pregnancy, it is revolutionizing the approach to prenatal diagnosis .
Women older than 35 years accounted for approximately 5% of the
maternal population but carried 30% of Down syndrome fetuses.
Serum screening methods were developed in the 1980s primarily to
screen low-risk women ،Highly sensitive screening using noninvasive
methods is understandably attractive to the maternal population, and by
reducing the need for invasive testing, it is probably saving lives .
For clinicians, the NT is particularly appealing as a screening technique
because it can be successfully measured on nearly all patients from 11
weeks to 13 weeks, 6 days of gestation .( Malone et al .,2001).
The pathophysiology of increased NT has been a matter of interest.
Several mechanisms have been proposed to explain the accumulation of
nuchal edema. Genes for collagen and matrix proteins are found on
chromosomes 21, 13, and 18. In trisomic fetuses, the excess ‘‘dose’’ of
chromatin leads to overproduction of these collagen subtypes in a doserelated
manner . (von Kaisenberg et al .,1998).
Histological and immunohistochemical studies demonstrated abnormal
endothelial development and impaired differentiation within the jugular
lymphatic sac tissue of aneuploid fetuses. Delayed endothelial remodeling
is proposed as the etiology for nuchal edema and distension of jugular
lymphatic sacs; poor endothelial cell migration and impaired cellular
adhesion are proposed as two possible mechanisms for the delay.
The hypothesis of endothelial cell dysfunction as the etiology of
increased NT isparticularly interesting, as it may provide an important
link between increased NT and cardiovascular defects. ( Matias et al .,
2005 ).